About Comets

What are comets?
Comets have been called "dirty snowballs." They are small celestial objects, made of ice, gas, dust, and a small amount of organic material, that orbit our Sun. There are about 1000 known comets and more are discovered each year.

What are the different parts of a comet?
Every comet has a nucleus, a stable, porous central mass of ice, gas, and dust that if often between 1 and 10 kilometers (0.6 to 6 miles) in size. The ice is made of varying amounts of water, carbon dioxide, ammonia, and methane. The dust may contain hydrogen, oxygen, carbon, nitrogen, silica, and some metals. The nucleus may have traces of hydrocarbons.

Nucleus of Comet Halley from the Giotto Project, European Space Agency. Note the jets of gas venting from the surface.Astronomy Picture of the Day

As comets approach our Sun [within about 450 million kilometers (280 million miles)], they heat up and the ice begins to sublimate (change from a solid directly to a gas). The gas (water vapor, carbon monoxide, carbon dioxide, and traces of other substances) and dust forms an “atmosphere” around the nucleus called a "coma." Material from the coma gets swept into the tail.

As comets move close to the Sun, they develop tails of dust and ionized gas. Comets have two main tails, a dust tail and a plasma tail. The dust tail appears whitish-yellow because it is made up of tiny particles — about the size of particles of smoke — that reflect sunlight. Dust tails are typically between 1 and 10 million kilometers (about 600,000 to 6 million miles) long. The plasma tail is often blue because it contains carbon monoxide ions. Solar ultraviolet light breaks down the gas molecules, causing them to glow. Plasma tails can stretch tens of millions of kilometers into space. Rarely, they are as long as 150 million kilometers (almost 100 million miles). A third tail of sodium has been observed on Comet Hale-Bopp.

Comets are enveloped in a broad, thin (sparse) hydrogen cloud that can extend for millions of kilometers. This envelope cannot be seen from Earth because its light is absorbed by our atmosphere, but it has been detected by spacecraft.

How are comets named?
Comets are named after the person who first reports their discovery. For example, Comet Halley is named for Edmund Halley, who determined that comets observed in 1531, 1607, and 1682 had essentially the same orbits and thus were a single comet. Based on his calculations, he correctly predicted the comet's return in 1758, but unfortunately, he did not live to see Comet Halley. Sometimes more than one person reports a new comet at the same time. In that case, the names are combined — as in the cases of Comet Hale-Bopp or Comet Shoemaker-Levy.

How big are comets?
A comet's nucleus is typically 1 to 10 kilometers (0.6 to 6 miles) across. The tail, however, can stretch for tens of millions of kilometers.

What are comets are made of?
Most of our information comes from studying the spectra of different comets. Scientists study the light reflected by different parts of a comet. Gases contain different elements. Each element (such as hydrogen), molecule (such as water), or ion (an electrically charged element or molecule) has a distinct pattern of emission or absorption that can be determined in the laboratory; this pattern is known as its spectrum. By matching patterns between laboratory measurements and comet observations, scientists can determine the composition of the comet.

Every comet is made of the same basic ingredients — ice and dust. However, comets probably vary in how much of the ice is water ice and how much is ice made of other substances, such as methane, ammonia, and carbon dioxide. Comets also vary in the different types of trace elements and hydrocarbons are present.

Several space missions, such as the European Space Agency's Giotto mission, have explored comets and provided detailed imagery of comet surfaces. A few missions are intended to sample comets. After a successful rendezvous with Comet Wild 2, NASA's Stardust mission will return comet dust and gas samples to Earth in January 2006. NASA's Deep Impact mission will encounter Comet Tempel 1 in July 2005, and will release a projectile into the comet surface to excavate a hole and expose a fresh surface on the nucleus. The spacecraft will collect data on comet emissions and will relay the data to scientists on Earth. While the data from these missions will be from only a few comets and might not be representative, the data will greatly improve our understanding of comet compositions.

What do the orbital paths of comets look like?
Based on observations of how comets move through the sky, scientists have determined that comets travel around our Sun in highly elliptical (oval-shaped) orbits. The time it takes to make a complete orbit is called a comet's period. Comet periods typically range from a few years to millions of years.

Where do comets come from?Comets are divided into short-period comets and long-period comets. Short period comets — such as Comet Halley — revolve around our Sun in orbits that take less than 200 years. Their orbital paths are close to the same plane of orbit as Earth and the other planets, and they orbit our Sun in the same direction as the planets. Based on these orbital characteristics, short-period comets are believed to originate in the Kuiper belt, a disk-shaped region extending beyond Neptune. The Kuiper belt contains small, icy planetary bodies, only a few of which have been imaged. These are the “leftovers” from early solar system formation. Occasionally the orbit of a Kuiper belt object will be disturbed by the interactions of the giant planets in such a way that it will have a close encounter with Neptune and either be flung out of the solar system or pushed into an orbit within our solar system.

Long period comets — such as Comet Hale-Bopp or Comet Hyakutake — take more than 200 years to orbit our Sun. Their orbital path is random in terms of direction and plane of orbit. Based on calculations from their observed paths, long-period comets are believed to originate in the Oort cloud. The Oort cloud is a spherical envelope that may extend 30 trillion kilometers (approximately 20 trillion miles) beyond our Sun. Oort cloud objects have never been imaged.

What happens when a comet approaches our Sun?
In the cold far reaches of our solar system, in the Kuiper belt and Oort cloud, comets are essentially just small chunks of ice and dust. Comets are nearly invisible except when they get close to our Sun.

As a comet approaches our Sun, it begins to heat up and the ice begins to sublimate — to change from a solid to a gas with no liquid stage. Some of the dust is left behind as the ice sublimates. It forms a dark, protective crust on the surface of the nucleus and slows the melting. In some places the protective layer is thinner, and jets of gas break through. The gas and dust form the cloud of the coma.

Our Sun emits a solar wind, a constant flow of gas and particles (mostly protons and electrons) that streams outward at 350 kilometers (about 220 miles) per second. Sunlight and solar wind sweep the dust and gas of the coma into trailing tails. Because sunlight and solar wind always flow outward from our Sun's surface, the tails always point away from our Sun no matter what direction the comet is moving in its orbit. This means that the tails can be in front of the comet as the comet moves away from our Sun on its return to the outer part of its orbit.

Two distinct tails develop — the plasma tail and the dust tail. The different shapes and angles of the tails are caused by the way different particles are affected by our Sun. The thinner, longer plasma tail forms a straight line extending from the comet. The particles in this ion tail are electrically charged and are pushed away from our Sun by the solar wind. The shorter dust tail is curved slightly. The larger particles in the dust tail do not have an electric charge and are not affected by the solar wind. Instead, the dust particles shed from the comet are repelled by the force of the sunlight and “lag behind” the comet in its movement around our Sun.

Comet tails get longer and more impressive as the comet gets closer to our Sun. As the comet approaches our Sun, it gets hotter and material is released more rapidly, producing a larger tail. Scientists estimate that a comet loses between 0.1 and 1 percent of its mass each time it orbits our Sun.

What happens when Earth passes through the path of a comet?
Meteor showers occur when Earth passes through the trail of dust and gas left by a comet along its elliptical orbit. The particles enter Earth's atmosphere and most burn up in a lively light show — a meteor shower. Some meteor showers, such as the Perseids in August and the Leonids in November, occur annually when Earth's orbit takes it through the debris path left along the comet's orbit. Comet Halley's trails are responsible for the Orionids meteor shower. For upcoming meteor showers and viewing suggestions, exploreStarDate's listing of the year's meteor showers.

Why are scientists interested in comets?
Scientists think that comets formed with our planets 4.5 billion years ago, so they contain important clues to the materials and processes in our early solar system. Scientists hope to learn even more about comets with NASA missions such as Stardust and Deep Impact and the European Space Agency's Rosetta mission.